1. Field of the Invention
The present invention relates to automatic focusing systems for use in optical pickup devices for scanning optical discs, for example. More particularly, the present invention relates to an automatic focusing system for carrying out a focusing operation by utilizing a moire image.
2. Description of the Related Art
In general, optical pickup devices for picking up information from optical discs are provided with an automatic focusing system to carry out a focusing operation.
For example, European Patent Publication No. 253,442 discloses a focusing system which detects focusing errors by means of a pair of diffraction gratings. In the system disclosed in the publication, the diffraction gratings are arranged one behind the other in a path of beam between an optical disc and a detection means so that the grating strips of one grating intersect those of the other grating at a small angle. Such an arrangement forms a moire pattern of bright and dark bands on the detection system. Since the moire pattern rotates according to the variation of the beam divergence, a focusing error can be detected by measuring the rotation of the moire pattern.
However, the focusing system disclosed in the publication has the problem of failing to detect focusing errors with a high precision because the amount of light received by the detection system greatly differs between in-focus condition and out-focus condition. Moreover, since the diffraction gratings must be arranged very accurately relative to each other in order to form a moire pattern, higher accuracy is required in manufacturing the system, thereby entailing a higher cost. Additionally, the focusing system still remains to be fully compacted.